Pantanal Temperature Excursion
You are right in saying that July falls in the Pantanal dry season:
the Pantanal has two seasons: a wet season from roughly October to March, and a dry season from April to September.
Nevertheless you should keep in mind that dry does not necessary mean warm. Indeed there are various websites warning that nights in the Pantanal can be quite colder than during the day:
Even during warmer seasons, the nights in the Pantanal can become crisp and cold. Therefore, tourists are urged to bring some warm clothing, regardless of when they are visiting.
The World Weather Online page on weather in the Pantanal shows that July is the coldest month, as both maximum and minimum temperatures graphs are at their minimums:
This is also backed up by the Pantanal Escapes webpage on climate, and the Lonely Planet webpage on the topic. Note that the temperature difference between the two sites might be due to a number of factors, including sampling years and location (it will be colder in the middle of the Pantanal, as opposed to the rural settlements).
This is all to say that you should consider a temperature excursion ΔT ≃ 10°C when packing clothes for a trip to the Pantanal.
What to Pack
When it comes to what to pack for cold weather, the answer is simply: warm clothes. Of course only you can judge how cold you might be, since this is very subjective. In my opinion though it is always better to overestimate the cold than it is to underestimate. If your jacket is too warm you can always take it off. On the other hand you'll definitely be cold if you have nothing warmer to wear. If you want to reduce your pack weight you can always invest in tactical lightweight gear. To this purpose, light polar fleeces are a very wise investment in my opinion.
A number of discussions on the topic exist online. For example, this post on the Tripadvisor forum explicitly mentions bringing warm clothing when visiting the Pantanal in July:
the evenings gets actually quite cold by Brazilian standards. I was there last year July (which is officially winter time) and misjudged myself how cold it actually was. If you go on the river on a cloudy day, it gets really cold also - wind chill and so on. So take at least one fairly warm jacket, like a windbreaker or polar fleece.
For a more general list of suggestions based on hands-on experience, see this virtual tourist discussion on the topic.